1. Field of the Invention
This invention generally relates to a line-type thermal printhead for use in a thermal printer of the direct printing type employing heat-sensitive paper or of the transfer type employing heat-sensitive ink ribbon and plain paper, and, in particular, to a driving system of a line-type thermal printhead in which the entire scanning line is driven as divided in blocks sequentially from one end to the other.
2. Description of the prior art
A line-type thermal printhead is well known in the art and it is commonly used in a thermal printer of the direct printing type in which use is made of heat-sensitive paper on which printing is carried out directly by the thermal printhead and of the transfer printing type in which use is made of heat-sensitive ink ribbon which is inserted between the thermal printhead and plain paper to effect printing on the plain paper. As is well known in the art, such a line-type thermal printhead includes a plurality of heat-producing elements, typically comprised of electrical resistors, arranged in the form of a single array as spaced apart from each other at a predetermined pitch, through which electrical current is selectively passed under control of a driving system in accordance with an image signal thereby printing a dot-form image on a recording medium.
In such a line-type thermal printhead, the plurality of heat-producing elements arranged in the form of a single line are divided into a plurality of blocks and the blocks, each including a predetermined number of heat-producing elements, are activated one after another in sequence. FIG. 2 illustrates how such a block driving of a line-type thermal printhead is carried out. In FIG. 2, there are shown only two print lines, i.e., previous line and current line; however, it is to be noted that, as is well known in the art, since a sheet of recording paper is typically moved relative to the thermal print-head in the direction normal to the line of arrangement of the heat-producing elements of the thermal printhead, there are, in fact, printed a number of print lines in front of the current print line. In FIG. 2, each circle indicates a pixel which is to be printed by the corresponding heat-producing element of the thermal printhead, and, as also shown in FIG. 2, the entire scan line is divided into a plurality of blocks and the blocks are driven one after another from one end to the other, in which the heat-producing elements of one block are selectively activated in accordance with the corresponding portion of an image signal.
In FIG. 2, the end dots of one block are indicated as shaded circles in the current line. These block end dots are less affected thermally by the image information of previous line and the adjacent dots of the current line as compared with the other dots which are not block end dots. And, thus, these block end dots tend to be lower in temperature when activated, which causes locally decreased image density thereby resulting in a printed image of poor quality. Such a disadvantage becomes particularly pronounced when thermal printing is effected in a half-tone mode, e.g., a multi-density level modulation mode (in the case of direct printing type) or an area modulation mode (in the case of transfer printing type).